In a diesel common rail power-train system, as the one sketched in FIG. 1, great importance has the fuel pressure run in the common rail placed upstream to the engine injectors, in order to assure efficiency and regularity of the engine operation. Such a fuel pressure within the rail is usually controlled by means of an electronic digital control unit that regulates, usually with a digital PWM (Pulse Width Modulation) technique, the metering valve unit controlling the fuel intake in the high pressure pump feeding the rail. The electronic control unit generally receives as inputs at least the monitored (digitized) battery voltage signal and the engine rotary speed signal, properly acquired with known detectors, and provides as output a PWM signal, with a required duty-cycle.
But, as known in the art, in a diesel common-rail power-train system, the battery voltage signal, that for generic purposes might be considered quite flat, is indeed affected by voltage drops and noises that cannot be disregarded when a PWM regulation of a high sensitivity load, such as the metering valve unit, is carried out by an electronic (digital) control unit. In fact, a common-rail power-train system usually comprises a digital control unit for driving a number of actuators, on the basis of digital signals coming from a number of relevant detectors, as well as on the basis of the monitored battery voltage signal. Among the actuators, as already mentioned, the metering valve unit is usually regulated by means of a PWM (Pulse Width Modulation) technique, by a proper controller module.
In such an environment, it should be pointed out that driving of many actuators (e.g., the fuel injectors) in a diesel common-rail power-train system is generally synchronous with the engine position (i.e., with the engine rotary speed) and thus it introduces a generally periodic effect on the battery voltage signal. More in detail, such a generally periodic effect on the battery voltage signal can be described as follows.
The battery voltage drops with conducted and irradiated noise through the electrical circuit of the power-train system generate a periodic ripple that superimposes the voltage mean value of the battery voltage signal, and the first harmonic frequency of the battery voltage signal, which is defined by its mean value plus the periodic ripple, is directly linked to the engine rotary speed.
Aliasing in battery voltage monitoring can thus affect the regulation of the metering valve unit by an electronic control unit, when sampling frequency matches the battery voltage signal harmonic spectrum, thus resulting in possible unduly pressure irregularities (oscillations) within the diesel common rail. In particular, considering a sampling time ts (e.g., 12.5 ms), since the afore-said periodic ripple superimposed on the battery voltage mean value can be considered to be a sub-multiple of the engine cycle period, digitalization of the battery voltage will result in a voltage signal substantially composed by the battery voltage mean value plus a low frequency component, having frequency equal to:
      f    aliasing    =                        1                  t          s                    -              rpm                  k          ·          60                        Where:rpm is the engine rotational speed expressed as revolutions per minute; −k is a constant depending on the cylinder number of the engine, defined as:
  k  =      2          Cylinder      ⁢                          ⁢      number      (e.g., k is equal to 0.5 for a 4 cylinder engine and it is equal to 0.125 for a 16 cylinder engine).
This means that when
            t      s        ≅                  k        ·        60            rpm        ,an additional low frequency component will be generated by aliasing effect, so that the digitalized form of the battery voltage signal will be affected by an equivalent harmonic spectrum not corresponding to the actual battery voltage signal.
In a diesel common-rail power-train system, the regulation of the metering valve unit, which is that valve metering the fuel intake volume to the high-pressure pump of the common rail, may be strongly affected by said aliasing effect in the battery voltage signal, mainly due to the fact that the metering valve unit regulation is carried out by the actuation of a PWM (Pulse Width Modulation) voltage. In fact, as can be easily ascertained, the duty-cycle (D*) of the PWM regulation of the metering valve unit, may be seen as:
      D    *    =            V      MU      *                      V        ~            batt      
Where V*MU is the desired mean voltage across the metering valve unit and {tilde over (V)}batt is the theoretical battery voltage (digital) signal.
Since the real battery voltage (Vbatt) can be seen as the sum of the theoretical battery voltage signal {tilde over (V)}batt with its voltage variation due to aliasing noise (ΔValiasing), and the voltage effectively applied (VMU) to the electromagnet of the metering valve unit results as the duty-cycle (D*) multiplied with the real battery voltage, one can easily ascertain that:
      V    MU    =                    D        *            ·              V        batt              =                  V        MU        *            ·                        V          batt                                      V            batt                    -                      Δ            ⁢                                                  ⁢                          V              aliasing                                          The mismatch between Vbatt and {tilde over (V)}batt due to the possible aliasing effect will results in an undesired noise affecting the metering valve unit regulation.
Applying the “small signal approximation” one can see that the noise oscillation in the metering valve unit voltage (ΔVMU) can be so approximate:
      Δ    ⁢                  ⁢          V      MU        =                    V        MU            -              V        MU        *              =                                        V            MU            *                                V            batt                          ·        Δ            ⁢                          ⁢              V        aliasing            Such a noise oscillation (ΔVMU) thus affects the metering valve unit regulation with an entity that depends on the proper transfer function used by the relevant controller module in the Electronic Control Unit in order to transform the nominal (desired) fuel intake volume request (Q*) of the high-pressure pump, in a duty-cycle set point for regulating said metering valve unit.
Adopting again the small signal approximation, the close loop scheme reported in FIG. 2 can describe the PWM control unit of the metering valve unit in a diesel common-rail power-train system, affected by the aliasing effect on the battery voltage signal.
In FIG. 2, one can see that Q* is the desired fuel intake quantity request; I(Q): is the I-Q characteristic of the metering valve unit (i.e. the characteristic curve showing the relationship between current (I)—fuel quantity (Q) in the metering valve unit); I*MU: is the electrical current required to meet the fuel intake quantity request (Q*) using a metering valve unit with I(Q) characteristic; ĨMU: is the nominal (theoretical) electrical current absorbed by the electromagnets of the metering valve unit; R(s): is the generic transfer function of the electronic control unit regulating the metering valve unit; V*MU: is the desired mean voltage across the electromagnet of the metering valve unit; ΔVMU: is the noise Oscillation in the metering valve unit voltage; and VMU: is the voltage effectively applied to the electromagnet of the metering valve unit.
In view of above, it should be clear that the voltage oscillation effect can be focused as an equivalent fuel quantity oscillation according to the following equation:
      Δ    ⁢                  ⁢          Q      aliasing        =      Q    ⁡          (                        Δ          ⁢                                          ⁢                      V            MU                                    R          ⁡                      (            s            )                              )      This leads to the equivalent scheme of FIG. 3.
The applicant has ascertained that ΔQaliasing could reach values up to ˜20÷40 mm3/stroke, with the aliasing frequency of the battery voltage signal ranging from approximately 1 to 3 Hz, resulting in a pressure oscillation on the common rail with a peak to peak magnitude directly proportional to its capacity, up to 15÷30 MPa. Such an undesired pressure oscillation due to the aliasing effect on the battery voltage monitoring, results in certain unevenness in the engine operation when a certain rotational speed of the same engine is reached, with possible bad consequences on the efficiency of the engine, its fuel consumption and performances.
Therefore, at least object is to solve the drawbacks of the actual diesel common-rail power-train system underlined above, by removing, or at least reducing, the aliasing effect on the battery voltage monitoring in a digital control unit for PWM (Pulse Width Modulation) regulations of a metering valve unit in a diesel common-rail power-train system. It is thus at least another object to provide a method to minimize common rail pressure irregularities due to aliasing effect on battery voltage monitoring in a digital control unit for PWM (Pulse Width Modulation) regulations of a metering valve unit in a diesel common-rail power-train system. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.